MPs were only able to avoid tax on their expense claims on the basis that they were “wholly, necessarily and exclusively” in relation to the performance of their Parliamentary duties.

However, senior tax inspectors are thought to have been alarmed when details of claims were disclosed by The Daily Telegraph which showed that MPs had been allowed to claim for luxurious goods and services.

MPs found to have claimed for non-essential items now face a tax bill of up to 40 percent on the value of such expensesexcessive claims. They may also have to pay interest and fines on the back-dated tax bills.

In May, HMRC wrote to all MPs asking if they wished to come forward and make voluntary payments. HMRC disclosed last night that it has now opened formal inquiries into 27 individual MPs.

It is thought that the authorities are also scrutinising MPs who avoided capital-gains tax when selling second homes; those who claimed for personal tax advice; and MPs’ travel claims for journeys between their homes and office if they do not live in their constituencies or London.

Last May, Hazel Blears voluntarily paid more than £13,000 to HMRC after being criticised for arranging her financial affairs in such a way to avoid capital-gains tax when selling her “second” home.

Senior accountants believe that HMRC would like to pursue a “high profile” tax avoidance case against an MP to demonstrate that politicians are subject to the same laws as the rest of the population.

John Whiting, tax policy director at the Chartered Institute of Taxation, said: “The Revenue will be looking carefully to see if the claims paid out are really reasonable and justified. And if not, MPs will have to pay tax on the value of those claims.

“I think that a lot of the more well-known claims that have emerged may well be judged not to be essential for an MP’s work.”

Mike Warburton, a tax partner at accountants Grant Thornton, added: “The rules are very clear and MPs will struggle to argue that claims like horse manure for their gardens are essential. HMRC will want to launch a high-profile case to put down a marker over what is acceptable.”

It is understood that Sir Thomas Legg, the former senior civil servant who is scrutinising MPs’ expense claims, has been made aware of HMRC’s inquiries. In a letter sent to MPs last week, he said he would not be studying the claims which are currently of interest to the tax authorities.

Tax inspectors closely followed the details of the Telegraph’s investigation into MPs expense claims earlier this year. The circulation of the Daily Telegraph in the Welsh town where the HMRC office which deals with MPs’ tax affairs is located, rose sharply at the time.

In June, Dave Hartnett, the permanent secretary for tax at HMRC, expressed fears that MPs may have broken the law.

He said that MPs often failed to pay the tax incurred after claiming back accountants’ fees for personal tax advice on their Commons allowances.

It was unclear how many had fallen foul of the rule, but Mr Hartnett said it was a common “mistake’”.

In May just five ministers out of 40 who had claimed the cost of an accountant on expenses confirmed to The Daily Telegraph that they had paid tax on the perk.

Appearing before the inquiry charged with making recommendations for reforming the expenses system, Mr Harnett was asked whether most MPs had paid the correct tax. “Many have, some haven’t,” he said. “Some have got their tax affairs correct and some haven’t. It is a mistake that gets made. They should all be paying tax.

“Over the last three months I have learned more about MPs’ expenses than I have over the rest of my career.”

Yesterday, Sir Christopher Kelly announced that he would publish his report setting out proposals for a complete overhaul of the expenses system on November 4th. Many MPs are braced for a crackdown on what they can claim in future. Sir Christopher is understood to be studying proposals for MPs to be allocated modest rented flats if they wish to stay in London.

Sir Christopher’s recommendations are expected to come just days before Sir Thomas Legg publishes his final report setting out his assessment of each MP’s claims over the past five years and whether they should repay money.

Sir Thomas last week wrote to MPs with his provisional conclusions. MPs have reacted in anger after it emerged he had decided to limit claims for gardening to a maximum of £1,000 a year since 2004 – with a cap of £2,000 for cleaning. Many MPs, including the Prime Minister, have had to repay thousands of pounds.

In a BBC interview to be broadcast today, John Bercow, the Speaker of the House of Commons, defended Sir Thomas’s inquiry.

He said the public had to be satisfied that MPs had “got the message” on expenses.

He added that he believed in the retrospective changes to the expenses rules, saying there had to be “consequences for past claims if they are shown to be wrong or extravagant”.